By John Egan, Kadant Black Clawson, Mason, OH

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In the paper industry, pulpers are often used in the stock prep, recycle, and paper machine processes. Hydrapulpers, which hydrate and defiber (break up into individual fibers) dry paper stock, are used to re-pulp bales of paper, re-slurry waste from the paper producing process, and recycle old corrugated cardboard and paper discarded at the mill. For all of these applications, a rotor is used to circulate the pulper slurry and provide the shear force required to fully defiber it. The rotor typically interacts with a perforated bedplate, and together the two determine the level of pulper circulation and shear force imparted to the material for a given pulper tub design. Because the fluid flow in a pulper is difficult to visualize, engineers do not fully understand how the pulper’s mechanical components contribute to the process of fully defibering a slurry. Most of today’s rotors were designed decades ago, but because they offer stable performance and reliability in a critical production-driven application, these rotors remain in service with little demand placed on suppliers to improve them.

At Kadant Black Clawson, CFD has played an integral role in recent efforts to improve the efficiency of their Hydrapulper rotor design. Engineers began by analyzing existing designs to understand their flow characteristics. They then evaluated a number of alternative designs (using a proprietary simplified rotor performance program) in hopes of identifying improvements that would result in more efficient operation. Several promising new “first cut” designs were developed, and detailed solid geometries of them were created and meshed using I-DEAS from SDRC. Models with 1.5 to 2 million cells were imported into FIDAP, where solutions were carried out in the fixed frame of reference. Flow in the rotating frame of reference was viewed in Fieldview (a product of Intelligent Light). The ability to view the results in both the rotating and stationary frames provided unique insights that led to a number of useful rotor performance improvements.

Computer model of an 8-ft pulper with a new low-power rotor

A CFD analysis was performed for each of the first-cut rotor designs. Using a stereolithography rapid prototyping process, a series of modular rotors was built and tested in order to evaluate the accuracy of the CFD predictions for these designs. Accurate calculation of power was critical to the success of the modeling efforts. In particular, the net power (equal to the total pulper power minus the mechanical power loss, rotor “underside” power, and stock shredding/defibering power) was computed for each case. Predictions for net power were found to be within 3% of measured values.

Transformation of velocity vectors to the rotating frame of reference (using Fieldview) was critical to refining the rotor design

The next step was building a fullsize version of the best design and testing it in a production 20-foot diameter, Kadant Black Clawson 10D Hydrapulper. A new 82-inch diameter rotor was built using the design developed with the aid of CFD to replace a conventional rotor in one of two identical pulpers running in parallel. Trials were run in batch mode, and performance (under stable operating conditions) was compared for the new and conventional rotors. Scaled up model results predicted a net power draw that was within 3% of the actual power consumption. The actual power savings based on a series of continuous trials, comparing the performance of both the new and conventional rotors installed in parallel pulpers, was found to be 25%. Just as important, the client has reported that defibering and circulation performance using the new rotor is at least equal to that of the conventional rotor.

An 81.6-inch-diameter Vortech rotor installed in a 10D Hydrapulper

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